Bruchmechanische Untersuchung von Metall/Keramik-Verbundsystemen für die Anwendung in der Hochtemperatur-Brennstoffzelle

Kuhn, Bernd

Dissertation / PhD Thesis/Book

Bruchmechanische Untersuchung von Metall/Keramik-Verbundsystemen für die Anwendung in der Hochtemperatur-Brennstoffzelle

Kuhn, B. (Corresponding author)FZJ*

2009
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-592-0

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment 50, 117 S. (2009) = Univ. Clausthal, Diss. 2009

Please use a persistent id in citations: http://hdl.handle.net/2128/3707

Abstract: Die Forschung und Entwicklung im Bereich der Hochtemperaturbrennstoffzelle (SOFC) ist traditionell von der Steigerung der elektrochemischen Leistungsfähigkeit dominiert. Mit der geplanten Nutzung der SOFC in größeren Einheiten (sog. Brennstoffzellenstapeln), vor allem für mobile Systeme in Personenkraftwagen und Nutzfahrzeugen rücken auch mechanische Aspekte ins Zentrum des Interesses. Dauerhafte mechanische Integrität sowie Zuverlässigkeit unter betriebsbedingt raschen Temperaturänderungen sind wichtige Voraussetzungen für den zufriedenstellenden Betrieb. Den mittels Reaktionslöten an Luft (Reactive Air Brazing) hergestellten metallbasierten Fügeverbindungen zwischen den keramischen Brennstoffzellen und den metallischen Gehäusekomponenten kommt dabei eine Schlüsselrolle zu. Aus diesem Grunde widmet sich die vorliegende Arbeit der Ergründung grundlegender Verformungs- und Schädigungsmechanismen im Fügeverbund als Grundvoraussetzung für eine zielgerichtete Weiterentwicklung des Reaktionslötens an Luft.
In this paper, the deformation and damage behavior of silver-based ceramic/metal solder joints for SOFC stacks was thermo-mechanically investigated to gain a basic understanding of the mechanical properties of RAB (Reactive Air Brazed) joints. Practical experience with existing RAB-joints in full-scale SOFC stack application showed the requirement for improvement of their mechanical stability. Conventional CuO-containing RAB-solders (Ag8CuO and Ag8CuOTiH2; mol-%) were compared to CuO-free braze-systems. Specimen geometries, which reproduce stack-like loading condition, have been selected for thermo-mechanical characterization. Deformation and damage models have been developed: In the as-brazed condition, the cell itself limits the mechanical performance of the joining system, because of its limited fracture energy under tensional load at room temperature. The integrity of the RAB-joint is governed by the fracture energy of the reaction layer that forms during the brazing process at the steel/braze-interface. Thermal aging leads to a reduction of joint adhesion and may endanger secure operation of the SOFC stack over many thermal cycles. CuO-free solders form reaction layers of higher chemical and mechanical stability with high fracture energy in the as-brazed state that is retained even after thermal aging. With a pre-oxidation treatment of the steel surface prior to brazing, age-related deterioration of joint adhesion was reduced to an acceptable level. The ability to reduce thermally induced stress by creep-deformation is regarded to be the greatest advantage of metallic brazing. Creep of the silver matrix was demonstrated by high-temperature experiments in symmetric shear conguration. However, the interfaces proved to be the limiting components of the joining systems: If the joints were mechanically overloaded, they rapidly failed by interfacial delamination. Loading below the maximum shearstrength led to creep deformation of the metallic braze-matrix. The determined Norton stress exponents, describing secondary creep, were in good agreement with literature values. For small deformation (< 60 % shear deformation) the visco-plastic properties of the silver matrix can be regarded to be similar to that of pure silver. To describe the shear properties at high deformation levels (> 60 % shear deformation) a rheological model was engaged. All joints exhibited quasi non-newtonian behavior with a dependency of shear viscosity on shear displacement-rate. As a conclusion it can be stated that current RAB-solders have advantages compared to conventional glass-ceramics - especially for automotive application. However, because of insucient mechanical stability and interfacial failure tolerance, these benets may so far not come to full advantage. Thus further R&D in metallic brazing for SOFC application should focus on interfacial reactions and related failure mechanisms.

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